(1) Technical Field
The present disclosure relates to energy band structure of organic materials in organic electroluminescence (EL) elements.
(2) Description of Related Art
In recent years, display devices in which a plurality of organic EL elements are arrayed on a surface of a substrate have become popular.
An organic EL element has a structure in which at least a light-emitting layer is sandwiched between an anode and a cathode.
Typically, the difference between an energy level of the lowest unoccupied molecular orbital (LUMO) of an organic material in a light-emitting layer (hereinafter, “LUMO level”) and a Fermi level of a cathode material is large. Thus, the organic EL element has a configuration in which a functional layer (electron injection layer, electron transport layer, etc.) for supplying electrons to the light-emitting layer is between the light-emitting layer and the cathode. In particular, it is well-known that when a functional layer is used that includes a metal material that has a Fermi level close to the LUMO level of the light-emitting layer, favorable electron injectability can be achieved. As such a metal material, an alkali metal or an alkaline earth metal can be used (JP 2013-514665).
A functional layer that includes an alkali metal or an alkaline earth metal can, for example, be formed by co-evaporation of metal material and organic material, thereby doping the organic material with the metal material.
However, in a thin film formed by co-evaporation, it is difficult to equalize doping concentration of the metal material with high precision across the organic EL elements arrayed on the surface of the substrate. Because differences in doping concentration of the metal material causes differences in injectability of electrons in the functional layer, this effects current efficiency, lifespan, etc., of the organic EL elements. Therefore, when metal material doping concentration in the functional layer is not uniform across the organic EL elements arrayed on the surface of the substrate, a problem arises in that properties vary between the organic EL elements.